What is Galvayne's Groove?

Horses have teeth that grow through their entire lives.

At the age of about 10 a dark line appears on the upper corner incisor. It extends down the tooth and extends the full length by the time the horse is 20. Then it slowly fades away and vanishes by 30 (if the horse lives that long).

Galvayne's Groove is one of the tools we use to estimate the age of an unknown horse.